This invention relates to a method and system for controlling an internal combustion engine. More specifically, the invention relates to technology for determining the maximum braking torque spark timing of the combustion process of an internal combustion engine.
Traditionally, maximum braking torque timing was determined through spark sweep. Unless requested by an operating condition for delayed spark timing, almost every calibration point required a spark sweep to determine if the engine could be operated at the maximum braking torque timing condition. A certain degree of safety margin was needed to avoid pre-ignition or knock if the engine could not be operated at maximum braking torque timing. This open loop spark mapping required significant effort to achieve a satisfactory calibration.
Various closed-loop spark timing control schemes utilize cylinder pressure measurements or spark ionization sensing. Based on testing data observation, the peak pressure in such schemes usually occurs around 15 degrees after top dead center (ATDC) at maximum braking torque timing; the 50% mass fraction burned occurs between 7 and 9 degrees after top dead center at maximum braking torque timing. A separate algorithm controls pressure ratio management (PRM(10)) around 0.55 to obtain the maximum braking torque timing.
Since these criteria are based on observations and may change at different operating conditions, each algorithm needs some calibration. The required calibration makes the determination of maximum braking torque timing complicated and somewhat inaccurate. In addition, it is clear that the combustion process must be matched with the engine cylinder volume change to attain the best torque. Maximum braking torque timing conditions should thus be determined efficiently based on a variety of parameters, and the observed xe2x80x9cusualxe2x80x9d occurrence of peak pressure may not always apply.
The primary object of this invention is therefore to provide a more efficient method for determining the maximum braking torque timing in an internal combustion engine. The above object is accomplished according to the principle of this invention by first calculating the net pressure change in an engine cylinder during the combustion process. The second derivative of the net pressure change is then calculated, the result of which represents the acceleration of the net pressure change. The spark timing of the engine is then advanced or retarded until the maximum acceleration point of the net pressure change is aligned with top dead center. This spark timing represents the maximum braking torque timing of the engine.